1. Field of the Invention
This invention relates to a tandem axial face seal for installation in pumps used for pumping cryogenic media, in order to seal a driveshaft against an interior of the pump.
2. Description of Prior Art
In what follows, supercooled liquids, starting approximately at xe2x88x92100xc2x0 C., are understood to be cryogenic media, the components of air in the liquid state, for example, such as nitrogen (N2) at lower than xe2x88x92196xc2x0 C., oxygen (O2) at lower than xe2x88x92183xc2x0 C. and argon (Ar) at lower than xe2x88x92186xc2x0 C., as well as hydrogen (H2) at lower than xe2x88x92253xc2x0 C. In technical terms, the above mentioned elements in liquid form are called liquid nitrogen (LiN), liquid oxygen (Lox) and liquid argon (LAr) and liquid hydrogen (LH2). Such supercooled liquids are produced on a large industrial scale when atmospheric air is split into its components by cooling and cleaning or, in the case of hydrogen, water is split into its components. The individual pure and liquid components are then stored in special cryogenic tanks under atmospheric pressure and are transported by trucks which are equipped with special cryogenic tanks. A portion of the cryogenic liquid, which is close to the boiling state, evaporates continuously because of a certain unavoidable heat input from the environment. Over a sufficiently long period of time the entire contents of the tank evaporate at unchanged temperatures and with an increasing pressure in the interior of the tank. While transferring cryogenic liquids, for example by pumping from one tank into another, or also when removing liquid, for example for using in an industrial process, it is continuously necessary to battle the undesired evaporation of the liquid. The more cryogenic liquid that evaporates, the more that must be considered a loss. The pumps are particular weak points during the transfer. Upon entering the pump, the conveyed liquid is close to a boiling pressure. Therefore a cryogenic pump must be constructed so that it pumps with a comparatively high suction pressure, and so that the pressure does not fall in the interior of the pump, because in case of miscellaneous underpressures the aspirated liquid would immediately evaporate. The cavitations created can cause the pump to run dry and become damaged. Heat flows continuously and unavoidably into the pump through the housing and the driveshaft of the pump, so that the supercooled medium can easily evaporate, in particular into the chamber of the motor side of the pump wheel, where only the approximate suction pressure of the pump in the pump medium prevails.
Therefore the seal which seals the rotating driveshaft against the pump interior is surrounded by gas, not by liquid. This is a dry-running seal. These seals are customarily known as labyrinth seals. The locally evaporated pump medium at the suction pressure pushes into the seal from the inside of the pump and tries to flow along the driveshaft in the direction of the motor. To minimize this leakage, a filtered confining gas at a slightly lower pressure, for example lower by 0.2 bar than the suction pressure of the pump, is pumped from the motor side of the shaft into the labyrinth seal for building up a counterpressure. In connection with a small cryogenic pump of approximately 40 kW output, approximately 15 standard m3, for example m3 at atmospheric pressure, of nitrogen per hour are required as the confining gas, and with large pumps more than twice that amount is required, just to mention an order of magnitude. This method therefore has one disadvantage that it is necessary to employ a separate confining gas, namely preferably nitrogen, so that a complete unit with a tank, filter and pressure regulator is necessary. Secondly, the use of a confining gas has the inevitable result that the cryogenic process liquid to be pumped is contaminated by confining gas, even though only slightly. These contaminations are more and more objected to by the users and are no longer tolerated in some cases.
Axial face seals with a spiral groove surface have also been used as alternatives to labyrinth seals. On one of their seal ring surfaces, such seals have a number of flat depressions of only about half a hundredths of a millimeter, which lead outward in a spiral shape. If the seal ring with the spiral-shaped grooves rotates in such a rotating direction that the mouths extending obliquely with respect to the ring periphery point in the direction of rotation, then ambient gas enters into the grooves and a dynamic pressure is created between the sliding rings, which provides a permanent gas cushion between the sliding rings, so that these run with essentially no friction. However, it is still necessary to pump a confining gas into the chamber in which the seal is located in order to compensate the suction pressure of the pump. Thus this the disadvantages remain, namely the requirement for installations for making available, filtering and pumping the confining gas, as well as the contamination of the process liquid with confining gas.
It is one the object of this invention to produce a seal for pumps for cryogenic media which can operate without extraneous confining gas, so that contamination of the pumped medium is impossible, which runs essentially frictionless, and which achieves a long service life, which is unattainable so far.
This object is achieved with a seal for the driveshaft of a pump for pumping cryogenic liquids, which is designed as a tandem seal. A sliding ring, with sliding faces on both sides have spiral-shaped grooves terminating at the outer periphery, is fixedly mounted on the driveshaft. The sliding faces are each adjoined by a sliding ring, which on one side is tightly connected via a metal bellows with the pump housing to be sealed, and on the other side with the pump housing on the motor side.
The seal is shown in the drawings in a longitudinal section taken through the driveshaft of a cryogenic pump.